1. Field of the Invention
The present invention generally relates to Time Division Duplex (TDD) technology, and more particularly, the present invention relates to a flexible TDD method for improving the efficiency of a communication system.
2. Description of the Related Art
Mobile communication systems have evolved into high-speed, high-quality wireless packet data communication systems (such as 3GPP High Speed Packet Access (HSPA) and Long Term Evolution (LTE), 3GPP2 High Rate Packet Data (HRPD), Ultra Mobile Broadband (UMB), and IEEE 802.16e standard systems) to provide data and multimedia services that extend beyond the early voice-oriented services.
As a representative broadband radio communication standard, LTE adopts Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) in the uplink.
In order to be prepared against decoding failure occurring at initial transmission, LTE adopts Hybrid Automatic Repeat reQuest (HARQ) for retransmission of the decoding-failed data on the physical layer. HARQ is a technique in which, when decoding has failed, the receiver sends to the transmitter a Negative Acknowledgement (NACK) so that the transmitter can retransmit the decoding-failed data. If the data is decoded successfully, the receiver sends to the transmitter an Acknowledgement (ACK) so that the transmitter can send new data.
In the OFDM system, the modulation signal is mapped to a 2-dimensional resource, i.e. a time-frequency resource. The time resource is divided into OFDM symbols. The frequency resource is divided into tones orthogonal among each other. In the OFDM system, a minimum resource unit is a Resource Element (RE), which is defined by an OFDM symbol on the time axis and a tone on the frequency axis.
The physical channel is a physical layer channel for transmitting modulation symbols output by modulating one or more coded bit streams. In an Orthogonal Frequency Division Multiple Access (OFDMA) system, multiple physical channels can be configured according to the purpose of the information bit stream or the receiver. The rule of pairing the physical channel with the RE is referred to as mapping.